This invention relates to magnetic resonance image (MRI) imaging, and, more particularly, to methods and apparatus for assembling magnetized permanent magnetic blocks used for an MRI magnetic field generator.
A high uniform magnetic field is useful for using magnetic resonance image (MRI) and nuclear magnetic resonance (NMR) systems as a medical device or a chemical/biological device. At least some popular and low maintenance cost MRI systems currently available use a permanent magnet system that creates a middle range uniform field (0.2 to 0.5 Tesla) in a pre-determined space (imaging volume). A permanent magnet system usually uses multiple permanent magnet blocks such as NdFeB to form a single magnetic object (polepiece) and to achieve desire high uniform magnetic field in the imaging volume.
For a magnetic field generator for an MRI system that uses permanent magnets, the magnets used in such an apparatus are often formulated from a plurality of magnetized blocks. However, it is difficult to place un-magnetized material blocks on a yoke plate first and then magnetize each block. Therefore, in actual manufacturing, the blocks are fabricated and then magnetized. The magnetized blocks are then arranged on a yoke plate so that each of the magnet blocks has a same magnetic pole facing upward. A pole piece is then placed on the top of the magnetized blocks.
However, it is difficult to assemble permanent magnetic blocks together to form one single object for use on an MRI system with high uniformity and accuracy due to the nature of very large magnetic force acting upon with each other and with other magnetic objects (such as the pole piece). Obviously, the magnetic field uniformity for the assembled magnetic system is very much depended on the quality of assembled permanent magnetic blocks. Hence, the uniformity of magnetic block dimensions is important to the quality of the magnetic field, and thus to the quality of a magnetic field generator for an MRI device.